Transmissions (e.g., optical or radio-frequency waveforms) used in traditional free-space communications inherently achieve a certain level of privacy. However, in many situations, it is still possible to intercept these free-space transmissions using a conventional coherent or non-coherent receiver. For example, a conventional coherent receiver can be placed between the transmitter and the intended recipient, behind the intended recipient, or slightly off-axis from the intended recipient to intercept the free-space communications.
In an attempt to prevent the data in a transmission from being surreptitiously obtained, various approaches have been proposed for protecting the data within a free-space transmission. Two examples of these techniques are encryption and obfuscation. For instance, some examples of typical encryption techniques are the Advanced Encryption Standard (AES) specification and the RSA cryptosystem. In contrast to typical encryption techniques, which encode the transmitted data, obfuscation techniques generally obfuscate a data payload with noise using techniques such as spread spectrum and Gaussian Dither modulation. While these techniques for concealing data aim to prevent message recovery from an intercepted transmission, they do not prevent the transmission (i.e., the transmitted waveform) from being intercepted.